1. Field of the invention
This invention relates to a preventive maintenance device for a resin-molded transformer which is extensively employed for reception of high voltages and, more particularly, to a partial discharge detecting device for detecting, with high accuracy, partial electric discharges which are induced in a primary winding of a high voltage side of a transformer.
2. Description of the Related Art
FIG. 8 is a front view showing one example of a conventional three-phase resin-molded transformer, and FIG. 9 is a fragmentary cross section of the transformer of FIG. 8 with an explanatory diagram showing a conventional partial discharge detecting circuit together with a resin-molded winding. The three-phase resin-molded transformer 1 comprises a high voltage winding assembly 3 including U-phase, V-phase and W-phase high voltage windings 3U, 3V and 3W (hereinafter referred to as "high voltage windings 3" or "windings 3", when applicable), respectively, which are wound on a three-leg iron core 2, and low voltage windings 9 wound inside the high voltage windings 3. The windings 3 and 9 are held through insulators 18 between a pair of frames 19 such that the windings 3 and 9 are coaxially held with a main insulation gap length G therebetween, and such that a gap length g is provided between each of the low voltage windings 9 and the iron core 2.
The high voltage windings 3 are formed as follows. In each of the high voltage windings 3, a plurality of unit coils 11, each formed by lap-winding a foil conductor having a predetermined width, are stacked axially at predetermined intervals, and are then series-connected to one another with bridging lead wires 11A. The unit coils 11 thus connected are molded with or potted in epoxy resin 12. Both ends of the high voltage winding are connected through buried lead wires 13 and 14 to a buried fitting, so that the windings are three-phase-connected with a resin-molded interphase connection leader 4 having high voltage terminals 5U, 5V and 5W. Of the unit coils u1 through u12, the unit coils u6 and u7 have taps which are connected through buried tap lead wires 16 and 17 to buried tap terminal units 6 and 7, respectively. Each of the tap terminal units 6 and 7 comprises a plurality of tap terminals. A connecting conductor 8 is provided for a pair of tap terminal units 6 and 7, so as to selectively connect one of the tap terminals of the tap terminal unit 6 to one of the tap terminals of the tap terminal unit 7. That is, the high voltage windings 3 are tapped such that the winding ratio of the primary and secondary windings can be changed while under a zero load.
In the resin-molded transformer thus constructed, partial electric discharges can be measured as follows. As shown in FIG. 9, the high voltage terminal 5 is connected through a coupling capacitor 22 to a partial discharge detecting sensor 21, so that a discharge pulse in the charging current of the coupling capacitor 22 is detected as a voltage pulse or current pulse with the sensor 21. The detected pulse is amplified by an amplifier 23 to a predetermined level. The output of the amplifier 23 is applied to a measuring unit 24, so that the magnitude (or the amount of discharge) of the pulse and the frequency of occurrence of the pulse are measured. In order to detect the voltage pulse, the sensor 21 may comprise an impedance detecting element, such as a resistor, electrostatic capacitive element, or inductive element. In order to detect the current pulse, the sensor 21 may comprise a high frequency current transformer. The measuring unit 24 may comprise a waveform observing device with an amplifier, such as an oscilloscope or a partial discharge measuring device. The partial discharge detecting sensitivity is calibrated with a calibrating pulse generator 25 comprising a rectangular pulse generator 25 a in series with a capacitor 25b. The calibrating pulse generator 25 is connected between the high voltage terminal 5 and ground, and supplies a known discharge pulse to the high voltage terminal 5. The magnitude of the supplied pulse is measured with the measuring unit 24 to detect the amount of discharge. (cf., Electric Society, Standard Investigation Association Standard, JEC-204, for instance).
The high voltage windings 3 of the resin-molded transformer are molded as follows. The series circuits of the unit coils 11 and the buried lead wires 13, 14, 16 and 17 are molded with epoxy resin by vacuum or pressure impregnation so that no voids are formed in the resultant molding. The molding is thermally set to form a resin mold layer 12 containing the unit 11 coils and the buried lead wires 13, 14, 16 and 17. That is, the high voltage windings are sufficiently insulated to the extent that no partial discharges are induced by the operating voltage (hereinafter referred to as a "corona free" condition, when applicable). Accordingly, when testing for detecting whether or not a "corona free" condition is obtained, the partial discharge detecting sensitivity should be as high as possible. However, in general, the detecting sensitivity is set to 50 pico coulombs (PC) to eliminate the effects of external noise.
The partial discharges induced by exciting the high voltage windings 3 are roughly divided into an external corona in the surrounding air when the surface of the resin mold layer 12 is contaminated, for instance, with moisture or salt, and an internal partial discharge (hereinafter referred to as "a void corona", when applicable) inside the resin mold layer 12. The high voltage windings 3 are surrounded by a large air gap (such as the main insulation gap G) and, therefore, the electric field in the resin mold layer 12 is generally low. Further, the probability is low that the void corona will spread towards a grounded component as the external corona. The buried tap lead wires 16 and 17 are laid between the tap coils u6 and u7 and, therefore, the resin mold layer 12 may include a portion where the insulation distance between the buried lead wires and the tap coils is locally short. A shared voltage corresponding to one unit coil is applied to this portion. Hence, if there are bubbles, or the like, in the portion, then a void corona is formed therein. In addition, the resin mold layer includes a portion where the lead wires 13 and 14 are extended along the plurality of unit coils 11 to the inter-phase connection leader 4. The sum of shared voltages corresponding to the plurality of unit coils 11 is applied to this portion, so that void coronas are liable to take place between the unit coils 11. If void coronas take place in this portion, they may break the insulation and possibly short circuit the unit coils through the lead wires. Hence, in the resin-molded transformer with no-load switching taps, it is essential for the partial discharge detecting device to positively detect the void discharges which are induced near the tapped coils located near the middle of the winding.
The conventional partial discharge detecting device in which the sensor 21 is connected through the coupling capacitor 22 between the high voltage terminal of the high voltage windings 3 and ground can positively detect the external corona which is induced to spread from the surface of the resin mold layer towards the components at ground potential, such as the iron core and the low voltage windings. However, in the case where the void corona is induced between the unit coils and the buried lead wires, a pulse circuit is formed to allow the discharge pulse thereof to circulate through a small number of unit coils. In this case, the discharge pulse is small and returns to the sensor 21 through the small stray capacitance which the non-grounded high voltage windings 3 form with ground. Accordingly, the conventional partial discharge detecting device has a low detection sensitivity. Furthermore, because the device is adversely affected by external noise, it is rather difficult to detect the void corona.